The REST Rhizome import request now requires the 'id' and 'version'
query parameters to either both be supplied or neither, and fails if
they do not match the manifest that is supplied in the request body.
Added a test case for this.
Added a test case to ensure that if the 'id' and 'version' query
parameters cause a hit (already in store) then the response is sent
immediately without reading the request body.
Improve the documentation for the REST Rhizome import request.